R8500 definitely.
Better 2D, better FSAA, better performance then the Ti200, better DVD playback, better 3D feature set, natively supports HydraVision.
Both are nice overclocking cards, the GF3 Ti200 should be abe to hit the speeds of the regular GF3 at least and maybe Ti500 speeds... the R8500 overclocks very nicely also though.
The GF3 has somewhat better drivers, but ATi has improved in that regrad and the difference isnt dramatic anymore IMHO.
For the slightly higher price the R8500 simply offers a much more well rounded graphics card.